Microstructure Evolution and Properties of Resistance Spot Welding of Press-hardened Steels

Transcription

1 Microstructure Evolution and Properties of Resistance Spot Welding of Press-hardened Steels H. L. Yi & H. L. Cai Northeastern University State Key Lab of Rolling and Automation

2 Background PHS: Press-hardening steels RSW: Resistance spot welding

3 PHS: Press-hardening steels Ford: Transit BIW: 492.1kg PHS 8.19% EuroCarBody, October 22 24, 2013

4 Forming Press hardening

5 RSW: Resistance spot welding Failure mode Interfacial fracture (IF mode) --Detrimental effect on welds Pullout fracture (PF mode) --Desired mode

6 RSW: Resistance spot welding Failure mechanism Tensile shear 1 Driving force for IF mode ----Shear stress on interface 2 Resistance for IF mode ----Increasing FZ size ----Increasing FZ strength

7 RSW: Resistance spot welding Failure mechanism Tensile shear 1 Driving force for PF mode ----Bending moment & tensile of HAZ and BM 2 Driving PF mode ----Decreasing strength of HAZ or BM

8 RSW: Resistance spot welding Failure mechanism Cross tensile 1 Driving force for IF mode ----Tensile stress on interface ----Fracture mode: Mode I 2 Resistance for IF mode ----Increasing facture toughness (K FZ IC )

9 RSW: Resistance spot welding Cross tensile Failure mechanism 1 Driving force for PF mode ----Shear stress on FAZ 2 Failure position for PE mode ----Maximum stress in HAZ by FE modeling 3 Driving for PF mode: ----Decreasing strength of HAZ ----Decreasing strength of FZ for increasing fracture toughness (K FZ IC )

10 RSW of PHS Materials: two 22MnB5 steels from industry RSW: welding parameters and properties Microstructures-properties relationship in spot welds Microstructural evolution in spot welds effected by reverse hardenability

11 Press-hardened steels Alloy composition in wt % C Mn Si Al Cr Mo Ti B P S PHS PHS

12 Hot stamping simulation Using a pilot hot stamping line with a flat die Hot stamping parameters of PHS1 ---Austenitizion: 925 o C for 3min ---Temperature of die surface: 20 o C ---Press: 60t (Pressure: 10MPa) ---Holding time: 20s Hot stamping parameters of PHS2 ---Austenitizion: 930 o C for 5min ---Temperature of die surface: 20 o C ---Press: 120t (Pressure: 30MPa) ---Holding time: 15s

13 Austenitization followed by air cooling In furnace Open furnace Air cooling

14 Die quenching with pressure A: Opening furnace B: Air cooling C: Contacting with lower die D: Contacting with upper die E: With pressure F: Air cooling

15 Mechanical properties of Presshardened steels Standard: GB/T (Straining rate: 0.67 X 10-3 s -1 ) 50mm gauge length sub-size specimens(1.5mm thick) UTS, YS(0.2% offset): reported by the machine TEL: Measured ΔL after fracture, TEL= ΔL / L (L = 50mm) Die temp / o C UTS / MPa YS / MPa TEL / % Microstructure PHS ± ±0.4 Full martensite PHS ± ±0.7 Full martensite

16 Microstructure of Press-hardened steels 5μm 5μm PHS1 PHS2

17 500nm 500nm PHS1

18 RSW of PHS1 Current Time Force Nugget Size TSS Optimized welding parameters: I=7.0~10.0kA, t=20~35cyc, F=6.5~10.5kN

19 RSW of PHS1: CTS & TSS Current Force TSS CTS K=F CTS /F TSS K:~0.3,Good

20 RSW of PHS2 TSS Tensile-Shear Fracture Load /kn Current Time Force F=9.0kN,t=20cyc Welding Current /ka 12 Tensile-Shear Fracture Load /kn F=9.0kN,I=9.5kA Welding Time /cyc 10.0 Nugget Diameter /mm I=9.5kA,t=20cyc Electrode Force /kn Nugget Size Nugget Diameter /mm F=9.0kN,t=20cyc Welding Current /ka Nugget Diameter /mm F=9.0kN,I=9.5kA Welding Time /cyc Nugget Diameter /mm I=9.0kA,t=20cyc Electrode Force /kn Optimized welding parameters: I=9.5~10.5kA, t=20~35cyc, F=8.5~9.5kN

21 8.5 RSW of PHS2: CTS & TSS Current 9.5 Force TSS CTS Tensile-Shear Fracture Load /kn Cross-Tensile Fracture Load /kn F=9.0kN,t=20cyc Welding Current /ka F=9.0kN,t=20cyc Nugget Diameter /mm Cross-Tensile Fracture Load /kn I=9.5kA,t=20cyc Electrode Force /kn I=9.5kA,t=20cyc K=F CTS /F TSS K:~0.3,Good Welding Current /ka Electrode Force /kn

22 Microstructure and hardness for PHS1 6 A B C D E A B C D E FZ A 485HV B 439HV C 300HV(Softening zone) HAZ D 428HV BM E 497HV

23 A-FZ 485HV B-HAZ 439HV 5μm C-Soft Zone 300HV E-BM 497HV

24 Microstructural evolution during RSW of PHS1 Temperature / o C T m A c3 A c1 γ α' Tempering γ+α γ+α' α' Tempering B C D A- A 熔核区 - FZ 热影响区 HAZ E- E 母材区 - BM Distance of center to the nugget /mm

25 Microstructure and hardness for PHS2 550 C A B D Vickers H ardness /HV A B C D Distance of Center to the Nugget /mm FZ A 488HV HAZ B C 435HV 440HV BM D 498HV

26 A-FZ 488HV B-HAZ 435HV 5μm 5μm E-BM 498HV 5μm

27 Conclusions Excellent mechanical properties performed in spot welds of press hardened steels: both tensile shear and cross tensile Good toughness (K=F CTS /F TSS ~0.3): softening zone & not so brittle in fusion zone due to autotempering Microstructural evolution: FZ by solidification and γ α' +tempering, HAZ_B by γ α' +tempering, HAZ_C by γ+α γ+α' which dual phase softening zone formed during heating but not cooling, HAZ_D by α' tempering Microstructure evolution in HAZ effected by the concept of reverse hardenability?